This invention relates to small programmable computers such as programmable desk-top calculators, personal computers and the like.
Programmable computers capable of storing a string of processing steps are very convenient when it is desired to store a string of computing steps or complicated processing steps which are difficult to achieve by manual operations, particularly a variety of decision steps.
BASIC (Beginners All-purpose Symbolic Instruction Code) is one of the most popular programming languages, comprising a plurality of lines each bearing one or more statements (one or more formulas or instructions having at least one meaning). When it is desired to draw a line of a program, the following input operation is executed. ##STR1## wherein ENTER indicates the completion of a line and is operated at the end of the line.
In the case where a line consists of two or more statements (this is called "multi-statement"), a colon (:) is placed between the statements. Integers from 1 through 99999 are usually used as line identifying numbers or indices for identification of the statements.
For example, in a program for evaluating selling amounts from unit price and quantity, the line numbers and statements are inputted in the following format.
______________________________________ 10 INPUT A , SURYO 20 LET AMOUNT = A *SURYO:DISP AMOUNT 30 END ______________________________________
Since the computer usually executes processes automatically begining with the lowest line number, the respective lines are numbered according to the order of its associated processes (for example, 10, . . . 20, . . . 30).
INPUT: this instruction implies numbers or characters to be introduced and what follows INPUT is a variant to be loaded and stored. PA1 LET: this implies the presence of a substitution sentence which is placed after LET. PA1 DISP: this implies a variant or a formula following DISP to be displayed.
Whereas the conventional programmable computer is capable of storing a string of processing steps accompanied by such indices as the line identifying numbers for retrieval of lines each consisting of one or more statements, it has the following drawback. If a new program line is inputted, then this new line will replace the previous one having the same line number. The operator may not be aware of such replacement. For example, if the operator makes error in inputting the line numbers, the contents of the program will disappear in whole or in part. This objectionable situation occurs when the operator misunderstands the indices or incorrectly operates keys.